Controlled Synthesis and Water Dispersibility of Hexagonal Phase NaGdF4:Ho3+/Yb3+ Nanoparticles

The luminescence properties of hexagonal sodium gadolinium fluoride nanoparticles co-doped with Ho3+ and Yb3+ (NaGdF4:Ho3+/Yb3+), prepared using a thermal decomposition synthesis route, were evaluated following 980 nm excitation. The synthesized nanoparticles were easily dispersed in nonpolar solvents, showed an extremely narrow particle distribution, and were determined to have a mean diameter of 15.6 +/- 1.2 nm. The predominantly green upconversion emission was observed to increase with increasing Ho3+ concentration due to an enhanced energy transfer (ET) from the neighboring Yb3+ ions. Post-synthesis, the nanoparticles were dispersed in water via modification of the capping oleic acid (OA) ligand. Upconversion emission intensity was observed to decrease upon dispersion in aqueous media likely due to an increase in nonradiative decay pathways. A total ligand exchange with poly(acrylic acid) (PAA) resulted in slightly more intense upconversion emission relative to oxidation of the OA to azelaic acid.